The present invention relates to a device for checking the conditions of the feed of a strip of wrapping material to a user machine, and more in particular it relates to an apparatus for feeding a strip of cigarette paper in a cigarette manufacturing machine.
As known, the checking devices of these machines are capable of detecting interruptions in the feed, e.g. as a consequence of the breakage of the paper strip, using a light-emitting element arranged to one side of the strip and a light-detecting element arranged to the opposite side; depending on the intensity of the light reaching the detector element, constituted e.g. by a phototransistor, the latter either conducts or is switched off, so as to provide information regarding any breakage of the strip.
It is therefore sufficient to check whether current flows in the output circuit of the phototransistor to acquire information about the presence or absence of the strip.
There are also other checking devices which are capable of detecting incorrect feed conditions, such as e.g. portions of tape having an irregular thickness as a consequence of a splicing of the terminal portion of a strip unwound from an almost empty reel with the initial portion of a strip unwound from a new reel.
Regarding the detection of such splices between the two strips, arising from reel changing operations, such splices area also detected by similar optical devices in order to be eliminated; the passage of a spliced portion at a checking device in fact completely cuts off the beam of light which filters through the paper strip in normal operating conditions.
In the known art, the above described optical detection devices are used in practice in the two alternative conducting and switched off conditions in order to maximize their reliability.
This however entails the use of separate checking devices to detect different conditions of anomalies in strip feed, and the sensitivity of the detector elements may furthermore vary as the local lighting conditions vary.